There are technologies to calculate an intake air amount of a cylinder in an internal combustion engine and control fuel injection amount and/or ignition timings based on the intake air amount of the cylinder. In these cases, the intake air amount of the cylinder is an air amount actually trapped in the cylinder and concerning combustion in the cylinder. Thus, accuracies in controlling the fuel injection amount and the ignition timings can be improved by employing the intake air amount of the cylinder as a parameter. A calculation method of the intake air amount of the cylinder is based on a rotation speed of engine and an intake pipe pressure.
Currently, many internal combustion engines provided with variable valve trains are put in practical uses. The variable valve trains adjust opening and closing timings of an intake valve and an exhaust valve to improve output power and fuel efficiency. In this case, internal EGR occurs by adjusting the opening and closing timings of the intake valve and the exhaust valve. Thus, the internal combustion engine provided with variable valve trains has an issue that the intake air amount of the cylinder cannot be calculated with accuracy, because unstable state of the internal EGR occurs by variations of the opening and closing timings of the valves, etc., even if the intake pipe pressure is constant. It can be considered to match every combination of parameters such as an operation range of engine, the opening and closing timings of the valves and so on to reflect the influence of the internal EGR. However, this causes another issue to increase matching processes and processing load.
As an air amount calculator for an internal combustion engine provided with variable valve trains, an art is known that is disclosed in U.S. Pat. No. 5,927,252 (JP-09-303242-A), for instance. In this art, an airflow meter detects the intake air amount in an ignition timing control, and the intake air amount of the cylinder is obtained as an average of the intake air amounts at opening and closing timings of the intake valve or as the intake air amount at a middle timing while the intake valve opens. Then, the ignition timings are calculated according to the intake air amount of the cylinder. However, the art is configured to derive the intake air amount of the cylinder basically from detection results of the airflow meter, and the ignition timing control is executed based on the intake air amount of the cylinder. Thus, this art has an issue that the intake air amount of the cylinder cannot be calculated with accuracy because of detection lags of the airflow meter and the passage lags of the intake air through an intake pipe.